Substantial increases in the efficiency of water usage for agricultural purposes have resulted from the introduction of drip (also known as trickle) irrigation systems, which first became feasible for large scale use with the availability of low cost plastic pipe. The crucial element in operation of these systems is the emitter, which must provide a selected low rate of flow (from one quart per hour or less to several gallons per hour or more) in response to a substantial (e.g. 5-50 psi) water pressure. In practice, the pipes for drip irrigation systems are generally laid down in matrix or grid patterns, with individual emitters being spaced apart so as to cover given operating diameters, typically from 2 feet to 6 feet. The installations may be above ground or under ground, but in either event hundreds to thousands of emitters are needed per acre. Properly operating systems are found to require only a fraction of the water typically needed with surface spray and irrigation techniques. The constant flow rate keeps the root area of plants moist but not oversaturated, and minimize evaporative and downward flow losses. At the same time chemical nutrients can be added to the soil and the soil itself can be used for beneficial filtering properties.
A number of emitter devices are currently known and in use on a commercial basis with drip irrigation systems. All of the more widely used devices are of the static type, in that they comprise passive flow constrictors which have a sufficiently high fluid impedance in the form of a small aperture to provide an adequate limitation of flow. All such devices are subject to clogging, because particle matter in the lines can block the small passageways. Most of the devices also have substantial operative disadvantages in terms of maintenance and replacement, whatever their design. One such device is built integrally with the pipe and must be cut out in order to be replaced if it becomes clogged. Another device overcomes this problem by being constructed of a number of replaceable parts, but is bulky and still requires manual handling in order to be cleaned.
The most significant limitations on present emitter devices, however, relate to their operating characteristics and cost. Most present emitters cost at least fifty cents each at the minimum, so that the total costs involved when utilizing typical emitter densities become very high. Such costs are not prohibitive for some installations, but do sharply restrict the economic usefulness of the drip irrigation technique. Moreover, in the present state of the art emitter devices operate only within a limited pressure range, as from 17 to 22 psi. The usage of a pressure regulator is of little practical benefit, because the long lines required for large area agricultural fields introduce substantial pressure variations along their length, either from variations in elevation, or from pressure losses along their lengths, or both. A difference of 100 feet in elevation is equivalent to a diminution in pressure from 50 down to 5 psi, an operating range which cannot be accommodated by existing emitter devices.